An action spectrum for melatonin suppression: evidence for a novel non‐rod, non‐cone photoreceptor system in humans

Melatonin suppression increased with irradiance in 215 light exposure trials conducted on 22 volunteers.

• Suppression of plasma melatonin was quantified using monochromatic light at various wavelengths and irradiances.
• At each tested wavelength, higher irradiance led to greater melatonin suppression.
• The constructed action spectrum demonstrated unique sensitivity to short wavelengths, distinct from established visual systems.
• The action spectrum did not align with known absorption spectra of rods and cones, indicating these photoreceptors likely do not play a major role.
• A strong fit was observed with rhodopsin templates, particularly one with a peak sensitivity at 459 nm.
• Findings suggest a novel short-wavelength photopigment may primarily mediate light-induced melatonin suppression in humans.

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